1. Field of the Invention
The invention relates to a method for transmitting data in a packet-oriented communications network with a guaranteed maximum transmission time for the data packets in the communications network, in particular in the communications network of a motor vehicle. Within the scope of this method, provision is made for a certain quality of service to be reserved, before data transmission, using a reservation request from the sender of the data to one or more recipients of the data, the reservation request comprising, parameters of the requested quality of service, in particular the data rate of the data stream and the latency, that is to say the maximum guaranteed transmission time, possibly in addition to further parameters.
2. Description of the Prior Art
Latency and transmission times, in particular, play a significant role when transmitting time-critical data in a communications network, which is also referred to as network for short and may be, in particular, an Ethernet network according to an Ethernet transport protocol. In this context, the new Ethernet Audio Video Bridging (AVB) standard (IEEE 802.1BA) is being examined with increasing interest for use in motor vehicles, which standard, in contrast to a standard Ethernet transport protocol, can provide guarantees for the maximum transmission time of data packets.
Ethernet AVB data can be transmitted using the IEEE 1722 or the IEEE 1733 transport protocol, in which case an Ethernet AVB network is also referred to as an AVB cloud inside which the guarantees are issued. Within the scope of the standard Ethernet AVB configuration two network nodes, which are referred to as the sender and the recipient and between which data is exchanged, can be at most 7 hops apart inside such an AVB cloud. In this case, the transmission of the data stream from one subscriber device to a next subscriber device in the communications network is referred to as a hop. The subscriber devices are able to be not only the sender and the recipient of the data stream but also buffers, which are referred to as switches, to receive and buffer a data stream and emit the data stream again for forwarding to the actual recipient, in which case each hop naturally costs transmission time. For this reason, a maximum of 7 such intermediate transmissions up to the final recipient are allowed within the Ethernet AVB standard.
Before the data is transmitted via a special communication connection, also called a communication route or path, a certain quality of service of the communication connection, in particular its latency and data rate, is reserved using a reservation request from the sender to the recipient. For this purpose, within the scope of the Ethernet AVB standard, so-called MSRP messages (Multiple Stream Reservation Protocol) are transmitted by the IEEE 802.1Qat protocol from the sender, referred to as the talker, to the recipient, referred to as the listener, via a maximum of 6 switches, that is to say 7 hops. These reservation requests, which are also referred to as signaling messages, also contain, in addition to parameters for the data rate of the data stream, a so-called AVB or QoS class, in which case two classes (class A and class B) are provided within the scope of the Ethernet AVB standard. Class A guarantees a maximum transmission time (latency) from the sender to the recipient of 2 ms. In contrast, class B guarantees a maximum transmission time of 50 ms. Further classes are currently not provided in the standard.
The emitted reservation requests are forwarded, via the switches, to the recipient, which processes the reservation request and confirms it in the case of a positive check. The confirmation of the reservation request is then returned by the sender, via the switches, to the recipient. which then emits the data according to the quality of service requested in the reservation request.
Depending on the AVB class, a usage time (presentation time) is associated with the actual data in the data packet used for data transmission in the header of the data packet and is concomitantly transmitted, which usage time, depending on the class, contains the time at which a recipient can evaluate the data, which time is determined in the synchronized AVB Ethernet communications network. In this case, the maximum transmission time stipulated depending on the class is taken into account, with the result that the current system time plus the maximum transmission time depending on the AVB class, that is to say 2 ms for class A or 50 ms for class B, is usually given as the usage time. As a result, within the synchronized communications network, all data received by devices within the AVB cloud can be used at the same time if synchronized operation of a plurality of subscriber devices in the network is required.
Since, depending on the topology of the network and length of the communication connection, that is to say the number of hops in particular, different subscriber devices receive the data packets at different times, the data must be internally buffered until the intended use at the usage time, which, depending on the data type, requires considerable memory in the subscriber devices in the communications network. Since, in contrast to products in typical IT environments, cost-optimized subscriber devices are intended to be used, in particular for use in automobile networks, a large memory is necessary in the subscriber devices, which unnecessarily makes these devices more expensive.
Another matter of expense within the scope of these networks is the communication performance of the switches which receive, buffer and emit the data streams. For this purpose, processing stacks referred to as queues are provided in the switches, into which stacks the data is placed and processed in succession for emission. In order to prioritize the data differently, a plurality of, often four, different queues of different priorities are usually provided in the switches, which queues are given a different amount of transmission time when data is present, with the result that data from the queue with the higher priority is emitted more quickly than data from the queue with lower priority. Data which is processed within the scope of the Ethernet AVB standard usually enter the queue with a higher priority on account of the maximum transmission time guaranteed by the Ethernet AVB standard so that said data is preferably processed before other data. However, the network topology and, in particular, the position of the sender and recipient in the network, which may be a different number of hops away from one another in particular, are not taken into account in this case.
US 2010/0080111 A1 discloses a method for the Ethernet switching of audio/video data using an Ethernet AVB transport protocol, in which the connection length of the audio/video systems is taken into account and the transceivers of the physical layer (PHY) are dynamically configured according to the connection lengths, which are generally short in comparison with conventional Ethernet networks, in order to adapt them to noise in the network which is less severe, in particular in the case of short connection lengths. This configuration makes it possible to use more favorable transceivers. However, the transmission method is based on the respective standards in this case and does not bring any time-saving and/or resource-saving transmission of the data in the network.
WO 2006/110960 A1 describes a method for transmitting digital data, in particular video and/or audio data, via an Ethernet network, the devices participating in the communication being synchronized to a local time. All data packets interchanged via the network are provided with a time stamp, with the result that the data packets can be sorted using the time stamp. It is also possible to delay the playback of the data packets until a common time or to specify particular playback times. Within a QoS service, the synchronization data can preferentially be transported to achieve the fastest possible synchronization over the entire network. However, this may result in problems if the preferred transport paths are overloaded by applications which must not use the preferred transport path at all in order to achieve the latency.